In the past, various methods have been employed for preparing surface-inactive catalysts. Several techniques may be used to increase the relative ratio of intra-crystalline acid sites to surface active sites. This ratio increases with crystal size due to geometric relationship between volume and superficial surface area. Deposition of carbonaceous materials by coke formation can also shift the effective ratio. Enhanced effectiveness is observed where the surface acid sites of small crystal zeolites are reacted with a chemisorbed organic base or the like.
Catalysts of low surface activity can be obtained by using medium pore zeolites of small crystal size that have been selectively deactivated by basic compounds, such as bulky amines and/or phosphines having an effective critical dimension or cross section diameter of about 6-7 Angstrom or greater. Another surface modification technique is deactivation by treating with metal compounds.
Zeolite catalysts having active pore acidic sites and inactive surfaces are of particular interest in shape selective catalysis, such as oligomerization of olefins. Recent work in the field of olefin upgrading has resulted in catalytic processes for converting lower olefins to heavier hydrocarbons. Heavy distillate and lubricant range hydrocarbons can be synthesized over ZSM-5 type catalysts at elevated temperature and pressure to provide a product having substantially linear molecular conformations due to the ellipsoidal shape selectivity of certain medium pore catalysts.
Conversion of olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed made up of medium pore zeolites having the structure of ZSM-5, In U.S. Pat. Nos. 4,150,062, 4,211,640, 4,227,992 and 4,547,613 (Garwood et al) disclose the operating conditions for various process techniques for selective conversion of C.sub.3.sup.+ olefins to mainly aliphatic hydrocarbons.
The final molecular conformation product of shape selective oligomerization is influenced by the pore structure of the catalyst. For the higher carbon numbers, the structure is primarily a methyl-branched straight olefinic chain, with the maximum cross section of the chain limited by the 5.4.times.5.6 Angstrom dimension of the largest ZSM-5 pore. Although emphasis is placed on the normal 1-alkenes as feedstocks, other lower olefins such as 2-butene or isobutylene, are readily employed as starting materials due to rapid isomerization over the acidic zeolite catalyst. At conditions chosen to maximize heavy distillate and lubricant range products (C.sub.20.sup.+) the raw aliphatic product is essentially mono-olefinic. Overall branching is not extensive, with most branches being methyl at about one branch per eight or more atoms.
Useful oligomerization catalysts may be made by treatment with organic silicon compounds, as described in U.S. Pat. Nos. 4,100,215 and 4,002,697 (Chen) to impart the desired degree of surface deactivation while being essentially free of carbonaceous deposits. Such treatment involves contacting the catalyst with a silane surface modifying agent capable of deactivating catalytic (acidic) sites located on the external surface of the zeolite by chemisorption. Other disclosures relating to shape selective oligomerization catalysts include U.S. Pat. Nos. 4,520,221, 4,568,768 and 4,658,079 (Chen et al).
It is a main object of this invention to provide an improved process for upgrading olefins to valuable lubricant quality product. Significantly improved linearity can be achieved by employing a catalyst comprising a medium pore shape selective siliceous metallosilicate with a substantially inactive surface.